Reducing NOx emissions has been a common environmental goal for many industries which use hydrocarbon fuels as a means of supplying heat and energy for various process needs. Typically a hydrocarbon fuel is combusted with an oxidant to release the fuel's chemical energy, as such single or multiple high temperature flames are also formed. Heat from the flame is transferred to process loads for a variety of purposes, such as to produce high pressure steam for electricity generation or to melt raw materials for making glasses and metals. The flame combustion process generates high temperature flue gas streams which contain major combustion species such as CO2 and H2O, as well as undesirable emissions such as SO2 and NOx (by which is meant NO, NO2, N2O, and any combination of two or all three of them). Normally, NOx in the flue gases of hydrocarbon combustion contains NO.
A number of technologies have been developed to reduce NOx emissions. These technologies include fuel staging, oxidant staging, reburning, oxy-fuel combustion, specially designed low NOx burners, selective non-catalytic reduction (SNCR), selective catalytic reduction (SCR), and flue gas scrubbing. Selecting a NOx reduction technology for commercial needs among many other factors, depends on the level of NOx to be reduced and its associated costs. Occasionally, plant space available to install the NOx reduction equipment also becomes a factor to consider in decision making. Furthermore, different types of NOx reduction technologies may be combined to reach targeted NOx emission levels.
In regenerative glass melting furnaces firing natural gas with high temperature preheated combustion air, the flame temperature becomes very high, which causes high NOx emissions. One way to reduce the NOx emission is to reduce the flame temperature by reducing the combustion air preheat temperature. Such a method, however, reduces the amount of heat recovered from the furnace flue gas.
If there is no additional heat recovery device downstream of the regenerator flue duct, the higher temperature flue gas then exhausts into the stack and imposes a fuel penalty and thus higher operating costs. Since flue gas exiting from the system has high temperature, it becomes important to make sure that downstream flue duct refractory and pollution control equipment can operate in these higher temperature regimes. There is a need to reduce NOx emissions from high temperature furnaces without reducing heat recovery efficiency.